The present invention relates to the fabrication and repair of superalloy components, specifically to a method of bonding superalloys at an interface.
Superalloys are employed in articles such as gas turbine hot section components because they exhibit high strength at high temperatures. Typical superalloys are nickel-based superalloys (such as INCONEL 617 and RENE 80) or cobalt-based superalloys (such as X-40 and FSX-414). Iron-based superalloys (such as V-57) are also common Frequently, superalloy components intended for use in gas turbine engines are fabricated as subcomponents that are joined together to form the final engine component. In some cases, the components can be joined by welding, however, in many cases either due to the inaccessibility of the joint to be bonded or the sensitivity of the superalloy microstructure to welding temperatures (such as with gamma prime precipitation hardened alloys), welding is not practicable. Accordingly, numerous diffusion brazing alloys have been developed to permit high strength bonding of these non-weldable components.
Diffusion brazing relies on the solid-state diffusion of atoms across an interface of the joint between the brazing alloy and the base metal. It necessarily follows that the diffusion brazing alloys are formulated to complement the base material of the parts being joined. Diffusion brazing alloys are thus generally nickel, iron, or cobalt-based alloys, depending on the composition of the base metal, combined with one or more melting point depressants such as boron or silicon. Brazing compounds thus have a composition similar to the base alloy but a melting point that is below that of the base metal. Brazing compounds are typically provided in the form of a powder, paste, or thin foil. The bonding of a joint is effected by placing the braze material on the joint and heating the joint to a temperature above the melting point of the brazing alloy but below the incipient melting point of the base alloy. The brazing material is drawn through capillary action into the joint and, upon cooling, forms a strong metallic bond across the joint.
Brazing is not without its disadvantages. Where the parts being joined have small features such as grooves or passages, the brazing alloy often wicks into and partially or completely obstructs these features. For example, the cooling panels in the transition ducts of an advanced high temperature industrial gas turbine have small cross-section cooling passages. The cooling panels are conventionally manufactured by milling a series of channels in a superalloy sheet and brazing a superalloy cover sheet over the milled channels. Conventional process controls have proved ineffectual in preventing the braze alloy from wicking into the cooling passages resulting in a high incidence of rejected parts.
Another disadvantage of brazing is that, because the braze alloy has a lower melting point than the surrounding base material, in extremely high temperature applications, the braze alloy will soften at a temperature lower than that of the surrounding part. The temperature limitation of the braze alloy therefore limits the operating temperature of the whole assembly. Post brazing heat treating improves to some extent the high temperature properties of brazed joints by diffusing the melting point depressants out of the brazed joint and into the surrounding base metal.
What is needed therefore is a method of diffusion bonding superalloy components without the use of brazing alloys.
The present invention comprises a method of diffusion bonding superalloy substrates by depositing an activator directly on the surface of the joint to be bonded and thereafter subjecting the joint to heat and pressure, which causes the surface of the superalloy, in the presence of the activator, to diffusion bond without the use of a brazing alloy. By eliminating the brazing alloy, a high strength, high temperature bond is achieved, yet there is no molten brazing alloy to be drawn through capillary action into any fine features surrounding the joint being bonded, and there is no residue left at the interface that would diminish the mechanical properties of the joint.